Accumulator using internal and external threads

ABSTRACT

An accumulator (ACC) in which a container main body ( 1 ) and a closure member ( 5 ) are screw-joined together. A reverse-buttress internal thread or an internal thread having an included angle of 90 degrees is used as an internal thread of the screw-joined portion. The reverse-buttress internal thread is formed in such a way that the inclination angle (β) of a clearance flank of the buttress internal thread is the inclination angle (β) of a reverse pressure flank ( 20 A) receiving a load, the inclination angle (θ) of a pressure flank is the inclination angle (θ) of a reverse clearance flank ( 2 B). The internal thread having an included angle of 90 degrees is formed in such a way that inclination angles (α),(γ) of both flanks are equal and the included angle (δ) is about 90 degrees.

TECHNICAL FIELD

This invention relates to an internal thread and an accumulator usingthe same.

TECHNICAL BACKGROUND

An accumulator is formed by dividing the interior of a container mainbody into a gas chamber and a liquid chamber by a bladder, screwing aclosure member into a top end portion of the container main body, andscrewing a supply/discharge tube into the other end portion thereof inthese screwing operations, threads stipulated by JIS (JapaneseIndustrial Standards) and ISO (International Organization forStandardization), for example, triangular threads are used. The angle ofinclination of a pressure flank and a clearance flank of each of thesetriangular threads is formed to be 30 degrees respectively, and anincluded angle is 60 degrees.

When an inner pressure in this accumulator increases, the closure memberis pressed outward, so that an axial and circumferential load, namely, aso-called fluctuating load is imparted to the thread repeatedly in therange of zero to a maximum level. This load is not borne uniformly byeach ridge but one-sided greatly in the tensile direction. Therefore,stress concentration occurs in a bottom of a valley of a base endportion of the internal thread which receives a large tensile load, sothat the internal thread is broken from this portion thereof.

In order to solve this problem, a buttress internal thread is usedinstead of the above-mentioned triangular thread.

The buttress internal thread has a shape of a combination of those of asquare thread and a triangular thread. Each internal thread ridge isprovided on one side surface thereof with a pressure flank whichreceives a load of an external thread, and on the other side surfacethereof with a clearance flank.

The pressure flank of the buttress internal thread is formed to have anangle of inclination of 7°, and the clearance flank thereof is formed tohave an angle of inclination of 45°.

When an inner pressure increases in an accumulator using this buttressinternal thread, a closure member is raised, and an external threadcomes into pressure contact with a pressure flank of the internal threadto raise the same. This raising force exerted on the pressure flank isdispersed, and generates a force for expanding a container main bodyoutward, this is a so-called diameter expansion force.

This diameter expansion force becomes small when the angle ofinclination of the pressure flank is small. For example, when the angleof inclination is 7°, the diameter expansion force becomes about ⅕ ascompared with a case where the angle of inclination is 30°. Therefore,since the diameter of the top end portion of the container main bodydoes not substantially expand, a load of the external thread imparted toa ridge of the internal thread cannot be made to escape to a ridge of asubsequent internal thread, and then the internal thread is broken.Especially, a large load is imparted to the first to third ridges of theinternal thread on the side of the base end thereof, and specially, alarge tensile stress occur in a bottom of a valley of the internalthread, so that the internal thread tends to be broken from this portionthereof in most cases. Accordingly, this causes the lifetime of theaccumulator to be shortened. The present invention aims at improving thelifetime of an accumulator in view of the above-described circumstances.Another object of the present invention is to prevent the breakage of aninternal thread.

DISCLOSURE OF THE INVENTION

The accumulator according to the present invention has abladder-containing container main body, a supply/discharge tube providedat one end of the container main body, and an internal thread providedat the other end thereof and screwed into a closure member, wherein theinternal thread is a reverse-buttress internal thread formed byreversing a clearance flank and a pressure flank of a buttress internalthread, or the internal thread is an internal thread of an includedangle of 90 degrees having an equal angle of inclination of theabove-mentioned two flanks.

The internal thread according to the present invention is an internalthread having a clearance flank and a pressure flank, wherein theinternal thread has an included angle of 90 degrees in which the anglesof inclination of the two flanks are equal to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 to FIG. 4 are drawings showing a first embodiment of the presentinvention, in which FIG. 1 is an enlarged view of a principal portion ofwhat is shown in FIG. 3,

FIG. 2 a longitudinal sectional view showing the condition of areverse-buttress internal thread of FIG. 1 which is receiving a load,

FIG. 3 an enlarged sectional view of a principal portion of what isshown in FIG. 4, and

FIG. 4 a longitudinal sectional view showing an accumulator.

FIG. 5 is an enlarged longitudinal sectional view showing a secondembodiment of the present invention,

FIG. 6 an enlarged longitudinal sectional view showing a thirdembodiment of the present invention,

FIG. 7 an enlarged longitudinal sectional view showing a fourthembodiment of the present invention, and

FIG. 8 to FIG. 10 drawings showing a fifth embodiment of the presentinvention, in which FIG. 8 is an enlarged view of a principal portion ofwhat is shown in FIG. 10,

FIG. 9 a longitudinal sectional view showing the condition of theinternal thread of FIG. 8 which is receiving a load, and

FIG. 10 an enlarged longitudinal sectional view showing a screwedportion of a container main body of the accumulator and a closuremember.

BEST MODE FOR CARRYING OUT THE INVENTION

The first embodiment of the present invention will be described withreference to FIG. 1 to FIG. 4.

An accumulator ACC contains a bladder 2 in a container main body 1thereof. This bladder 2 is a pleated bladder creased so that the bladderis folded to a predetermined shape. A flange portion 3 of this bladder 2is engaged with an upper portion 1 a of the container main body 1 andfixed by a closure member 5. This closure member 5 is provided with asupply/discharge port 6 communicating with the interior of the bladder2, and an external thread 21 engaged with an internal thread 20 of thecontainer main body 1. This internal thread 20 is a reverse-buttressinternal thread provided with twelve internal thread ridges F1 to F12.The internal thread ridge F1 is positioned on a base end portion 20D ofthe internal thread 20, and the internal thread ridge F12 on a top endportion 20W thereof.

In this reverse-buttress internal thread, the shape (=angle ofinclination=15 degrees, for example) of a pressure flank and the shape(=angle of inclination=45 degrees, for example) of a clearance flank ofbuttress internal thread ridges F1 to F12 are reversed with respect tothe other. The angle of inclination of the clearance flank is set to βof a load-receiving reverse pressure flank 20A, while the angle ofinclination of the pressure flank is set to θ of a reverse clearanceflank 20B.

Namely, the angle β of inclination of the reverse pressure flank 20A isequal to that 45° of the clearance flank, while the angle θ ofinclination of the reverse clearance flank is equal to that 15° of thepressure flank.

The reverse pressure flank 20A and reverse clearance flank 32B continueto each other via a bottom 20C of a valley of the internal thread, aradius r1 of this bottom 20C of the valley is set to 0.4 mm. The radiusr1 of the bottom of the valley of the internal thread is selectedsuitably as necessary, and a preferable range of the radius r1 is 1/10to ⅓ of the pitch of the thread.

The container main body 1 is provided in a bottom portion 1 b thereofwith a through hole 10, in which a supply/discharge tube 13 is fittedfirmly via an O-ring. A flange portion 14 of this supply/discharge tube13 is in pressure contact with a receiving portion 11 of the throughhole 10. A poppet valve 16 having a cushion cup 15 is supported slidablyin the supply/discharge tube 13. The supply/discharge tube 13 is fixedto the container main body 1 by a nut 17.

The operation of this embodiment will now be described.

The accumulator Acc is connected to a hydraulic circuit (not shown) viathe supply/discharge tube 13. When a liquid pressure in the hydrauliccircuit varies to cause the pressure in the container main body 1 toincrease, the closure member 5 is pressed in the direction of an arrowA5. As a result, as shown in FIG. 2, the ridge M1 of the external thread21 comes into pressure contact with the reverse pressure flank 20A ofthe internal thread 20.

At this time, a load imparted to the internal thread ridge F1 isdispersed to cause a pressing force in the direction of an arrow A20,i.e., a diameter expansion force to occur. Therefore, the reversepressure flank 20A is elastically deformed as the flank 20A slides on aside surface of the ridge M1 of the external thread 21, and the flank20A thereby expands outward. The angle of inclination β of this reversepressure flank 20A is 45° and as three times as large as that of 15° ofa pressure flank of a prior art internal thread, so that acircumferential displacement increases greatly. The portion of a loadwhich cannot be borne by the internal thread ridge F1 is made to escape,and imparted to a subsequent internal thread ridge F2. The portion ofthe load which cannot be borne by the internal thread ridge F2 is alsomade to escape, and imparted to a subsequent internal thread ridge F3.Such processes are repeated one after another, and a total load impartedto the external thread is transmitted to internal thread ridges F1 andthen to F12.

A second embodiment of the present invention will be described withreference to FIG. 5.

A difference between this embodiment and the first embodiment (FIG. 1 toFIG. 4) resides in that the radius of a bottom of a valley 20C of areverse-buttress internal thread is 0.21 mm.

A third embodiment of the present invention will be described withreference to FIG. 6.

The differences between this embodiment and the first embodiment (FIG. 1to FIG. 4) reside in that the angle β of inclination of a reversepressure flank 20A of a reverse-buttress internal thread is 50°; theangle β of inclination of a reverse clearance flank 20B is 10°; and theradius r1 of a bottom of a valley of the internal thread 20C is 0.21 mm.

A fourth embodiment of the present invention will be described withreference to FIG. 7.

A difference between this embodiment and the first embodiment (FIG. 1 toFIG. 4) resides in that a reverse-buttress internal thread is a steppedtapered internal thread. This internal thread is provided with a steppedportion 20× and a tapered portion 20Y. Internal thread ridges F1 to F3of this stepped portion 20X are summit-cut, and a line connecting thesesummits becomes a straight line L parallel to a center line 10C of anaccumulator. The internal thread ridges F3 to F12 of the tapered portion20Y are summit-cut, and a line connecting these summits becomes atapering line T inclining in the direction approaching the center line10C from the base end portion 20D toward the free end portion 20W.

The embodiments of the present invention are not limited to theabove-described embodiments. The embodiments may also be formed, forexample, in the following manner.

When a buttress internal thread in a standard arrangement condition, inwhich, for example, a free end thereof is positioned on the upper sidewith a base end thereof positioned on the lower side, and, in which aflank (load receiving surface) of a smaller angle of inclination ispositioned as a surface on the lower side of the internal thread ridgewith a flank of a larger angle of inclination positioned as a surface onthe upper side thereof, is set in a reversely arranged state in which,for example, the upper and lower portions of the buttress internalthread are reversed, and, in which the free end thereof is positioned onthe lower side with the base end thereof positioned on the upper side,the flank of a larger angle of inclination becomes a load-receivingreverse pressure flank with the flank of a smaller angle of inclinationbecoming a reverse clearance flank. In this manner, the buttressinternal thread may be formed to a reverse-buttress internal thread bysetting the buttress internal thread in a reversed state.

The angle β of inclination of the reverse pressure flank may be selectedsuitably from the levels in the range of 30° to 60° instead of being setto an angle equal to the angle of inclination of the clearance of thisbuttress internal thread.

The angle θ of inclination of the reverse clearance may be set to alevel different from that of an angle equal to the angle of inclinationof the pressure flank of the buttress internal thread instead of beingset to the angle equal to the angle of inclination of the pressureflank. The angle θ of inclination mentioned above is set smaller thanthat β of the reverse pressure flank. The angle θ is suitably selectedfrom the levels in the range of 0° to 150. For example, when the angle βof inclination of the reverse pressure flank is 45°, the angle θ ofinclination employed of the reverse clearance flank is 0°.

EXPERIMENTAL EXAMPLES

Concerning the internal thread of the above-described embodiment, atriangular internal thread with an angle of 600 and a buttress internalthread, a tensile load was calculated by a finite-element methodassuming that a total load imparted to the external thread is 332620N,and a maximum tensile stress (N/mm²) of the bottom of the valley of eachof the internal thread ridges F1-F12 was determined to obtain theresults shown in the following Table 1.

In this Table 1,

-   -   a reference letter (F no) denotes an internal thread ridge        number,    -   a reference letter (A) denotes a reverse-buttress internal        thread (first embodiment) having an angle of inclination of a        reverse clearance flank of 15°, an angle of inclination of a        reverse pressure flank of 45°, and a radius of a bottom of a        valley of the internal thread of 0.4 mm,    -   a reference letter (B) denotes a reverse-buttress internal        thread (second embodiment) having an angle of inclination of a        reverse clearance flank of 15°, an angle of inclination of a        reverse pressure flank of 45°, and a radius of a bottom of a        valley of the internal thread of 0.21 mm,    -   a reference letter (C) denotes a reverse-buttress internal        thread (third embodiment) having an angle of inclination of a        reverse clearance flank of 10°, an angle of inclination of a        reverse pressure flank of 50° and a radius of a bottom of a        valley of the internal thread of 0.21 mm,    -   a reference letter (D) denotes a stepped tapered        reverse-buttress internal thread (fourth embodiment) having an        angle of inclination of a reverse clearance flank of 15°, an        angle of inclination of a reverse pressure flank of 45°, and a        radius of a bottom of a valley of the internal thread of 0.4 mm,    -   a reference letter (E) denotes a prior art buttress internal        thread having an angle of inclination of a clearance flank of        7°, an angle of inclination of a pressure flank of 45°, and a        radius of a bottom of a valley of the internal thread of 0.21        mm, and

a reference letter (F) denotes a triangular internal thread formed tohave an angle of inclination of a pressure flank of 30° and an angle ofinclination of a clearance flank of 30°, respectively. TABLE 1 MAXIMUMTENSILE STRESS (N/mm²) OF A BOTTOM OF A VALLEY OF AN INTERNAL THREAD Fno A B C D E F 1 317 449 435 355 846 629 2 355 373 467 349 385 466 3 262458 319 393 393 458 4 171 166 264 287 299 264 5 173 192 259 243 257 2546 140 161 247 364 199 222 7 146 160 197 169 185 194 8 138 151 191 138201 191 9 139 137 134 141 136 142 10 148 148 161 145 105 124 11 138 127139 142 94 129 12 134 126 137 135 85 96

The following became clear from what were shown in this Table 1.

(1) The tensile stress of the bottoms of the valleys of the internalthread ridges F1 to F3 on a base end portion of the internal threads islarger as compared with that of the other internal thread ridges F4 toF12.

(2) The tensile stress of the bottoms of the valleys of thereverse-buttress internal threads (A), (B), (C), (D) is smaller ascompared with that of the other internal threads (E), (F).

(3) The tensile stress of the bottom of the valley of thereverse-buttress internal thread (A) is the smallest among the internalthreads (A), (B), (C), (D).

(4) The tensile stress of the bottom of the valley of thereverse-buttress internal thread is extremely small, i.e., about 1/2.8of that of the corresponding portion of a buttress internal thread (E).

Experimental Example 2

Next, the circumferential displacement (mm) of the internal threadridges F₁ to F₁₂ of the above-mentioned (B) (=reverse-buttress internalthread of the second embodiment), (C) (=reverse-buttress internal threadof the third embodiment) and (F)(=regular metric thread) was determinedto obtain the results as shown in Table 2. TABLE 2 CIRCUMFERENTIALDISPLACEMENT (mm) F no B C F 1 0.0212 0.0245 0.0144 2 0.0211 0.02450.0134 3 0.0210 0.0245 0.0130 4 0.0209 0.0245 0.0132 5 0.0213 0.02480.0133 6 0.0218 0.0254 0.0137 7 0.0225 0.0260 0.0141 8 0.0231 0.02660.0144 9 0.0240 0.0273 0.0150 10 0.0253 0.0286 0.0157 11 0.0264 0.03000.0166 12 0.0280 0.0314 0.0179

The following were clarified from what were shown in this Table 2.

(1) The circumferential displacement tends to increase as the internalthread ridge number increases.

(2) The circumferential displacement of the reverse-buttress internalthreads (B), (C) is larger than that of the regular internal thread (F).

(3) The circumferential displacement of the reverse-buttress internalthread (C) is larger than that of the reverse-buttress internal thread(B).

A fifth embodiment of the present invention will be described withreference to FIG. 8 to FIG. 10.

The difference between this embodiment and the first embodiment residesin that, as a means for screwing a container main body and a closuremember on each other, an internal thread of an included angle of 90degrees having an equal angle of inclination of two flanks is usedinstead of a reverse-buttress internal thread.

Namely, as shown in FIG. 4, an accumulator ACC contains a bladder 2 in acontainer main body 1 thereof. This bladder 2 is a pleated bladder 2creased so that the bladder is folded to a predetermined shape. A flangeportion 3 of this bladder 2 is engaged with an upper portion 1 a of thecontainer main body 1 and fixed by a closure member 5. This closuremember 5 is provided with a supply/discharge port 6 communicating withthe interior of the bladder 2 and an external thread 21 engaged with aninternal thread 20 of the container main body 1. This internal thread 20is provided with twelve internal thread ridges F1 to F12. The internalthread ridge F1 is positioned on the base end portion 20D of theinternal thread 20, and the internal thread ridge F12 on the top endportion 20W thereof.

This internal thread is an internal thread having an included angle of90 degrees. The included angle δ is 90 degrees, and larger than that ofa standard triangular thread, for example, an ISO standard triangularthread (ISO 261). An angle α of inclination of a pressure flank 20 athereof and that γ of a clearance flank 20 b are formed to have an equallevel of 45 degrees. Since these two flanks 20 a and 20 b have an equalangle α, γ, the flanks become symmetric, this internal thread can be cuteasily as compared with a reverse-buttress internal thread. Namely, itis not necessary to prepare a cutting tool of a special shape, and acutting process can be carried out efficiently.

Incidentally, though the angles α, γ of inclination of the flanks 20 a,20 b are formed to be equal to each other, the included angle δ may notnecessarily be set to 90 degrees accurately. The included angle includesangles in the neighborhood of 90 degrees. Namely, the included angle δis set suitably and selectively to 90 degrees or to a level in the rangeof substantially 90 degrees. For example, the angles α, γ may be set to44.5 degrees respectively, and the included angle δ to 89 degrees.

The pressure flank 20 a and clearance flank 32 b continue to each othervia a bottom 20 c of an internal thread valley, and the radius r1 ofthis bottom 20 c of the valley is set to 0.4 mm. The radius r1 of thebottom of the thread valley is set suitably and selectively asnecessary, and preferably in the range of 1/10 to ⅓ of the pitch of thethread.

The container main body 1 is provided at a bottom portion 1 b thereofwith a through hole 10, in which a supply/discharge tube 13 is fittedfirmly via an O-ring. A flange portion 14 of this supply/discharge tube13 is in pressure contact with a receiving portion 11 of the throughhole 10. A poppet valve 16 having cushion cup 15 is slidably supportedon the supply/discharge tube 13. The supply/discharge tube 13 is fixedto the container main body 1 by a nut 17.

The operation of this embodiment will now be described.

The accumulator Acc is connected to a hydraulic circuit (not shown) viathe supply/discharge tube 13. When a liquid pressure in the hydrauliccircuit varies to cause the pressure in the container main body toincrease, the closure member 5 is pressed in the direction of an arrowA5, and the external thread ridge M1 of the external thread 21 isthereby brought into pressure contact with the pressure flank 20 a ofthe internal thread 20 as shown in FIG. 9.

A load imparted to the internal thread ridge F1 is dispersed to cause apressing force in the direction of an arrow A20, i.e., a diameterexpansion force to occur. Therefore, the pressure flank 20 a iselastically deformed as the pressure flank 20 a slides on a side surfaceof an external thread ridge M1 of the external thread 21, the pressureflank 20 a thereby expanding outward.

During this time, since the angle α of inclination of the pressure flank20 a of the internal thread is 45°, which is 1.5 times as large as that(i.e., 30°) of the pressure flank of a prior art standard triangularthread, a circumferential displacement greatly increases.

Also, the angles α, γ of inclination of the two flanks 20 a, 20 b of theinternal thread are 45 degrees respectively, and the included angle δ is90 degrees, the thickness of a root portion and the strength of theinternal thread increase as compared with those of a standard triangularthread.

The portion of a load which cannot be borne by the internal thread ridgeF1 is made to escape and imparted to the subsequent internal threadridge F2. The portion of the load which cannot be borne by the internalthread ridge F2 is made to escape and imparted to the subsequentinternal thread ridge F3. Such processes are repeated one after another,and a total load imparted to the external thread is transmitted to theinternal thread ridges F1 to F12.

The embodiments of the present invention are not limited to theabove-described embodiments. For example, an internal thread having anincluded angle of 90 degrees may be formed to be a stepped taperedthread by providing a stepped portion and a tapered portion on theabove-mentioned internal thread of an included angle of 90 degrees inthe same manner as in the fourth embodiment (FIG. 7).

Since the internal thread having an included angle of 90 degrees isthick at a root portion thereof and has a high strength in comparisonwith a standard triangular thread, this internal thread can be utilizedfor a thread made of a fragile material, such as gypsum.

Advantageous Effect of the Invention

Since the accumulator according to the present invention is providedwith a reverse-buttress internal thread in which a clearance flank and apressure flank of a buttress internal thread are reversed, or providedwith an internal thread having an included angle of 90 degrees in whichthe angles of inclination of the two flanks are equal to each other, theangle of inclination of a load receiving surface increases greatly.Therefore, the diameter expansion force is increased in comparison withthe prior art examples, thereby a load borne by the thread ridges islessened. This prevents the breakage of the internal thread, and thelifetime of the accumulator can be improved.

Since the internal thread according to the present invention is aninternal thread having an included angle of 90 degrees in which theangle of inclination of the clearance flank and that of the pressureflank are equal to each other, the strength of the internal thread ishigh as compared with a standard triangular thread, so that the internalthread is not easily impaired. Moreover, since the internal thread iseasily cut as compared with a reverse-buttress internal thread, thecutting process efficiency can be improved.

1. An accumulator having a bladder-containing container main body, asupply/discharge tube provided at one end of the container main body,and an internal thread provided at the other end of the container mainbody and engaged with a closure member, characterized in that theinternal thread is a reverse-buttress internal thread in which aclearance flank and a pressure flank of a buttress internal thread arereversed, or the internal thread is an internal thread in which anincluded angle is 90 degrees having an equal angle of inclination of thetwo flanks.
 2. An accumulator according to claim 1, wherein thereverse-buttress internal thread is formed by putting a buttressinternal thread in a standard arranged state having a clearance flankand a pressure flank to a reversely arranged state, and thereby turningthe clearance flank into a load-receiving reverse pressure flank withthe pressure flank turned into a reverse clearance flank, wherein thereverse pressure flank expands outward when the reverse pressure flankreceives a pressing force.
 3. An accumulator according to claim 1,wherein the reverse-buttress internal thread is a thread in which areverse clearance flank and a load-receiving reverse pressure flank areformed by reversing the shape of the pressure flank of the buttressthread and the shape of the clearance flank thereof, wherein the reversepressure flank expands outward when the reverse pressure flank receivesa pressing force.
 4. An accumulator according to claim 1, wherein thereverse-buttress internal thread is a thread formed by setting an angleof inclination of a clearance flank of a buttress internal thread tothat of a load-receiving reverse pressure flank with the clearance flankformed into a reverse clearance flank, wherein the angle of inclinationof the reverse clearance flank is set to be smaller than that of thereverse pressure flank, and the reverse pressure flank expands outwardwhen the reverse pressure flank receives a pressing force.
 5. Anaccumulator according to claim 1, wherein the angle of inclination ofthe reverse pressure flank is 30° to 60°.
 6. An accumulator according toclaim 1, wherein the reverse pressure flank and reverse clearance flankcontinue to each other via a bottom of a valley of the internal thread,wherein the radius of the bottom of the valley of the internal threadbeing in the range of 1/10 to ⅓ of the pitch of the thread.
 7. Anaccumulator according to claim 1, wherein the angle of inclination ofthe reverse clearance flank is 0° to 15°.
 8. An accumulator according toclaim 1, wherein the reverse-buttress internal thread is provided with astepped portion and a tapered portion.
 9. An internal thread having aclearance flank and a pressure flank, characterized in that the internalthread has an included angle of 90 degrees in which an angle ofinclination of the two flanks is equal to each other.
 10. An internalthread according to claim 9, wherein the internal thread is providedwith a stepped portion and a tapered portion.
 11. An accumulatorcomprising: a bladder-containing container main body; a supply/dischargetube provided at a first end of the container main body; an internalthread provided at a second end of the container main body; and aclosure member engaged with the internal thread; wherein the internalthread is a reverse-buttress internal thread whereby each thread of theinternal thread includes a clearance flank facing the first end and apressure flank facing the second end, the clearance flank having aclearance angle of inclination of about 0° to 15° and the pressure flankhaving a pressure angle of inclination of about 30° to 60°.
 12. Anaccumulator according to claim 11, wherein: the pressure flank andclearance flank of adjacent threads continue to each other via a bottomof a valley of the internal thread; and a radius of the bottom of thevalley of the internal thread is approximately 1/10 to ⅓ of a pitch ofthe threads.
 13. An accumulator according to claim 11, wherein: thereverse-buttress internal thread is provided with a stepped portion anda tapered portion.
 14. An accumulator comprising: a bladder-containingcontainer main body; a supply/discharge tube provided at a first end ofthe container main body; an internal thread provided at a second end ofthe container main body; and a closure member engaged with the internalthread; wherein each thread of the internal thread includes a firstflank facing the first end and a second flank facing the second end, thefirst flank and the second flank of at least one thread of the internalthread meeting at an angle of approximately 90 degrees, with each flankhaving an equal angle of inclination.
 15. An accumulator according toclaim 14, wherein: the first flank and the second flank of adjacentthreads continue to each other via a bottom of a valley of the internalthread; and a radius of the bottom of the valley of the internal threadis approximately 1/10 to ⅓ of a pitch of the threads.
 16. An accumulatoraccording to claim 14, wherein: the internal thread is provided with astepped portion and a tapered portion.